Variable inductance coil form assembly

ABSTRACT

The assembly includes a coil form having a coil receiving portion and an internally threaded center hole extending longitudinally therethrough. The internally threaded surface has an axial groove therein extending along its length, and the groove is generally concentric to the threaded surface and has a depth greater than the depth of the thread. A thin flat resilient strip is positioned within the groove. The strip has a width that substantially occupies but is less than the width of the groove and is of a thickness that is less than the distance that the depth of the groove exceeds the depth of the thread.

United States Patent Hazen, III et al.

[4 1 Aug. 15, 1972 [54] VARIABLE INDUCTANCE COIL FORM ASSEMBLY [72] Inventors: Curtis Hazen, III, Andover; Gerd Achim Tuchen, Boxford, both of 21 Appl. No.: 116,527

3,259,862 7/1966 Richard ..336/1 36 3,281,744 10/1966 Melanson ..33 6/l36 X 3,400,627 9/1968 Raynovich ..85/33 X FOREIGN PATENTS OR APPLICATIONS 1,332,745 6/1963 France ..85/33 Primary Examiner-E. A. Goldberg Att0rneyR. J. Guenther and Edwin B. Cave [57] ABSTRACT The assembly includes a coil form having a coil receiving portion and an internally threaded center hole extending longitudinally therethrough. The internally threaded surface has an axial groove therein extending along its length, and the groove is generally concentric to the threaded surface and has a depth greater than the depth of the thread. A thin flat resilient strip is positioned within the groove. The strip has a width that substantially occupies but is less than the width of the groove and is of a thickness that is less than the distance that the depth of the groove exceeds the depth of the thread.

6 Claims, 4 Drawing Figures PATENTEDMIB 15 I972 H CURT/S ET MEMO GA. TUCHEN ATTOR/VFV VARIABLE INDUCTANCE COIL FORM ASSEMBLY FIELD OF INVENTION This invention relates to the field of electrical coil devices and within that field to coil forms adapted to receive a core member.

DESCRIPTION OF THE PRIOR ART It is common practice in the manufacture of inductance coils to provide a form on which the wire is wound and to provide the form with an axial center hole in which a high permeability core is positioned. The inductance can then be finely adjusted to a particular value by axially displacing the core within the core form.

To facilitate making this adjustment and to hold the core in position once the adjustment has been made, it is common to provide the core and the center hole of the coil form with mating threads. However, because high permeability cores and dimensionally stable coil forms are both made from brittle materials, thread interference is not permissible between the mating threads. Clearance must be provided between the threads, and as a result shock, vibration, or just a change in orientation of the coil form results in some movement of the core.

For a precision inductor, where the inductance is limited to a variation of less than l/ th of one percent, mating threads alone are therefore insufficient to provide the necessary positional control of the core. Other arrangements for achieving this control have been proposed and three such arrangements, which at first blush appear similar to the structure of this invention, will now be examined.

US. Pat. No. 3,l06,69l issued to Hisao Maeda discloses a variable inductance coil assembly including a tubular coil form having a longitudinal slot therein. A T-shaped insert of thermoplastic insulating material is positioned in the slot with the cross-bar portion of the insert placed in engagement with the exterior of the form and the shank of the insert projecting into the center hole of the form. The coil is thereafter wound on the form, the windings of the coil overlaying the crossbar portion to secure the insert in place, and an externally threaded ferrite core is screwed into the center hole of the form. The core has an outer diameter that is about the same as the inner diameter of the center hole,

and as a result the external threads of the core cut into and tap the shank of the insert. The threads formed in the insert conform closely to the threads of the core and provide the prevailing force that holds the core in position.

The problem with this arrangement is that experience with such plastic bodies indicates that they tend to lose this prevailing force over a period of time. Changes in humidity result in dimensional changes in the insert. In addition, changes in temperature result in a different thermal expansion in the insert from that of the core. Consequently, stresses are produced in the insert that, in combination with the stresses that are introduced by the initial cutting of the threads, cause the insert to cold flow. The holding forces that are exerted on the core therefore dissipate over a period of time, whereby the core is able to shift and cause a change in inductance.

Furthermore, because the crossbar portion of the insert positioned between the coil and the form is fon'ned of the same material, it is also dimensionally unstable. As a result, both the effective diameter of the coil and the wire tension of the coil change with time. These changes affect the electrical stability of the inductor and also cause changes in inductance.

US. Pat. No. 3,119,975 issued to Yoshikauzu Arita discloses a variable inductance magnetic core assembly including a tubular magnetic core having two or more longitudinal flutes formed in the wall of its center hole. Each flute occupies only a small arcuate portion of the circumference of the center hole and each flute has a flexible rack-like rod mounted therein. The threads of the rods are adapted to engage the external threads of an inductance varying cylindrical core positioned within the center hole, and each rod is bowed along its length so that its middle presses against the core and acts to retain the core in its selected position.

The problem with this arrangement is that the retaining force is limited to the middle of the inductor, essentially no retaining force being applied towards the ends of the inductor. This may be of minor importance where the inductance varying core occupies most of the length of the center hole, but in many inductors the core occupies considerably less than this. With such a core, the adjustment range of the inductor would be quite limited inasmuch as the core could not be moved very far from the middle of the inductor. In addition, the adjustment torque would vary as the core is displaced within this middle area.

Finally, US. Pat. No. 3,129,397 issued to G. A. Scherry discloses a variable inductance coil assembly in which the coil form comprises a cylindrical middle portion having a radially extending flange portion at each end. An internally threaded center hole extends longitudinally through the coil form, and each flange portion is provided with an axially offset cylindrical recess that opens into the center hole. A cylindrical resilient insert is press-fit into the recess, and a portion of the circumference of the insert projects into the center hole. Thus when an externally threaded core is screwed into the center hole, the threads of the core engage the outer surface of the insert. This engagement acts to retain the core in its selected position.

The problem with this arrangement is that because the insert has the same configuration as the recess, and is press-fit into the recess, its resiliency is substantially reduced, if not eliminated, and as stated in the patent, the core tends to cut threads into the insert rather than deflect it. It is then subject to the same deficiencies as discussed with respect to the first arrangement described.

Furthermore, because the core must always remain in engagement with most of the length of the insert if the desired retaining force is to be attained, not only must the core be of a length to extend within the flange portion of the coil form, but-the adjustment range of the core is limited to a fraction of the width of the flange portion. Finally, the adjustment torque varies with the length of engagement of the core with the insert.

SUMMARY OF THE INVENTION The coil form assembly of the present invention does not suffer from any of the deficiencies of the previously described devices. The assembly comprises a tubular coil form having an internally threaded center hole adapted to receive an externally threaded core. The center hole has a longitudinal groove formed in the threaded surface thereof that extends along the length of the form, the bottom of the groove being generally concentric to the threaded surface, and the groove having a depth greater than that of the thread. A thin flat resilient strip having a width that is less than the width of the groove and a thickness that is less than the distance that the groove depth exceeds the thread depth is positioned within the groove, and when the core is threaded into the coil form the strip is bowed. As a result, the strip exerts a force along the entire length of the core that presses the core into intimate contact with the portion of the thread opposite to the strip. The resulting friction holds the core in a selected position and prevents changes in inductance due to core slippage.

Because the resilient strip is flexed by the core rather than tapped, the force it exerts on the core does not diminish with the passage of time. Further, because the strip extends along the length of the coil form, both the retaining force and the adjustment torque are the same wherever the core is positioned. There is therefore complete freedom in selecting the length of the core and in adjusting the inductance with the selected core.

DESCRIPTION OF THE DRAWING FIG. 1 is a perspective view of a coil form assembly embodying the present invention;

FIG. 2.is an exploded perspective view of the coil form assembly;

FIG. 3 is a sectional view showing the assembly before a core is inserted; and

FIG. 4 is a sectional view showing the assembly after the core is inserted.

DETAILED DESCRIPTION OF THE INVENTION Referring to FIG. 1 of the drawing, an illustrativeembodiment of the coil form assembly includes a coil form comprising a cylindrical coil receiving portion 12 and a flange portion 14 at each end thereof. Each flange portion 14 includes a circular groove 16 extending around its periphery adjacent to the coil receiving portion 12. In addition, each flange portion 14 includes a pair of depending legs 18 for spacing the coil receiving portion 12 from the surface on which the coil form assembly is supported.

As clearly seen in FIG. 2, the coil form 10 consists of an upper half 20 and a lower half 22, the mating surfaces thereof lying along the diameter of the coil receiving portion 12. Registration between the two halves is achieved by a pair of protrusions 24 in the lower half 20 that are accommodated by a pair of conforming recesses (not shown) in the upper half 22.

The two halves 20 and 22 of the coil form 10 are secured together by a pair of terminal clips 26. Each tenninal clip 26 includes an arcuate spring portion 28 and a terminal portion 30. The spring portion 28 is accommodated in the groove 16 in the flange portion 14,

and the diametrically opposite interior surfaces adjacent to the ends of the spring portion are spaced somewhat closer together than the diameter at the base of the groove. The ends of the spring 28 are therefore deflected apart as the spring portion is positioned in the groove 16, and the resulting spring tension clamps the upper and lower halves 20 and 22 together. One end 29 of the spring portion 28 is elongated so as to serve as a termination for the coil wound on the coil receiving portion 12. The terminal portion 30 extends from the spring portion 28 and serves as a mounting terminal for the coil form 10.

An internally threaded center hole 32 extends through the coil form 10 concentric to the coil receiving portion 12, and a longitudinal groove 34 is formed in the threaded surface. The groove 34 extends along the entire length of the coil form 10 and the bottom surface is generally concentric to the threaded surface. In addition, the groove 34 has a depth that is greater than the depth of the thread and the groove subtends an angle of between 60 and degrees.

A thin, flat resilient strip 36 is positioned within the groove 34. The strip 36 is somewhat longer than the length of the groove 34 and therefore extends beyond at least one end of the coil form 10. Furthermore, as shown in FIG. 3 the strip 36 has a width that substantially occupies but is less than the width of the groove 34, and therefore the strip is not bowed by its being positioned within the groove. However, since the groove subtends an angle of at least 60, the strip forms a chord that at least intersects a continuation of the root of the internal thread. It is seen that as this angle and thereby the width of the groove 34 increases, the intersection between the strip 36 and the continuation of the internal thread also increases. Finally, the strip 36 has a thickness that is less than the distance that the depth of the groove exceeds the depth of the thread.

An externally threaded ferromagnetic core 38 is positioned within the center hole 32. The core 38 is of a diameter such that its threads engage the threads of the center hole 32, and the mating threads conform to one another to provide a noninterfering fit.

The core 38 is screwed into the coil form 10 by pressing one end of the core into engagement with the portion of the strip 36 that extends beyond the end of the coil form 10. The portion of the strip 36 adjacent to the end of the coil form 10 is thereby pressed down into the groove, and the external thread of the core 38 is able to engage the internal thread of the center hole 32. A tool is then inserted into a slot 40 in the end of the core 38 to advance the core into the center hole.

As shown in FIG. 4, the strip 36 is bowed to conform to the circumference of the core 38, and inasmuch as it is thinner than the distance between the crest of the cores thread and the bottom of the slot 34, it is not compressed against the bottom of the slot and tapped by the thread of the core. Consequently the strip 36 behaves solely like a flexed spring, and it exerts an upward force along the entire length of the core 38 that presses the external thread of the core and the internal thread of the hole 32 opposite to the strip into intimate engagement with one another. The resulting friction holds the core 38 in a selected position and prevents changes in inductance due to slippage of the core.

The force that the strip 36 exerts on the core 38 and therefore the torque that must be exerted to change the position of the core is approximately proportionalto the strip length engaged by the core. In addition, the force that the strip 36 exerts is approximately proportional to the third power of the thickness of the strip and to the Youngs modulus of the strip. The force, however, is virtually unaffected by the width of the strip 36.

The coil form is advantageously molded from a glass filled thermoset having a low thennal coefficient of expansion, such as phenolic compound. The strip 36 is advantageously made from a stable nonmetallic material such as the product polyethylene terephthalate sold under the trademark Mylar.

Although but one embodiment of the invention has been described, it will be understood that it is but illustrative and that various modifications may be made therein without departing from the scope and spirit of this invention as defined in the appended claims.

What is claimed is:

l. A variable inductance coil form assembly comprismg:

a coil form having a coil receiving portion and an internally threaded center hole extending longitudinally therethrough, the internally threaded surface within the coil receiving portion having an axial groove therein extending along its length, the groove having a depth greater than the depth of the thread; and

a thin flat resilient strip positioned within the groove, the strip having a width that is less than the width of the groove at the grooves widest point and having a thickness that is less than the distance that the depth of the groove exceeds the depth of the thread, the strip forming a chord that intersects a continuation of the root of the internal thread of the coil form.

2. A variable inductance coil form assembly as in claim 1 wherein the bottom of the groove is generally concentric to the internally threaded surface of the coil form.

3. A variable inductance coil form assembly as in claim 1 wherein the groove subtends an angle of between 60 and 4. A variable inductance coil form assembly as in claim 1 wherein the groove extends along the entire length of the coil form and the strip occupies the entire length of the groove.

5. A variable inductance coil form assembly comprismg:

a coil form having a coil receiving portion and an internally threaded center hole extending longitudinally therethrough, the internally threaded surface within the coil receiving portion having an axial groove therein extending along its entire length, the groove having a depth greater than the depth of the thread and subtending an angle of between 60 and 120, the bottom of the groove being generally concentric to the internally threaded surface of the coil form; and

a thin flat resilient strip positioned within the groove, the strip having a width that substantially occupies but is less than the width of the groove at the s st dbein fale ttht Z%3 rh" ngii i ti'ie groo v anci qa mg of a thickness that is less than the distance that the depth of the groove exceeds the depth of the thread, the strip forming a chord that intersects a continuation of the root of the internal thread of the coil form.

6. In combination;

a cylindrical member having an internally threaded center hold extending longitudinally therethrough, the internally threaded surface having an axial groove therein extending along its length, the groove having a depth greater than the depth of the thread; and

a thin flat resilient strip positioned within the groove, the strip having a width that is less than the width of the groove at the grooves widest point and having a thickness that is less than the distance that the depth of the groove exceeds the depth of the thread, the strip forming a chord that intersects a continuation of the root of the internal thread of the cylindrical member. 

1. A variable inductance coil form assembly comprising: a coil form having a coil receiving portion and an internally threaded center hole extending longitudinally therethrough, the internally threaded surface within the coil receiving portion having an axial groove therein extending along its length, the groove having a depth greater than the depth of the thread; and a thin flat resilient strip positioned within the groove, the strip having a width that is less than the width of the groove at the grooves widest point and having a thickness that is less than the distance That the depth of the groove exceeds the depth of the thread, the strip forming a chord that intersects a continuation of the root of the internal thread of the coil form.
 2. A variable inductance coil form assembly as in claim 1 wherein the bottom of the groove is generally concentric to the internally threaded surface of the coil form.
 3. A variable inductance coil form assembly as in claim 1 wherein the groove subtends an angle of between 60* and 120*.
 4. A variable inductance coil form assembly as in claim 1 wherein the groove extends along the entire length of the coil form and the strip occupies the entire length of the groove.
 5. A variable inductance coil form assembly comprising: a coil form having a coil receiving portion and an internally threaded center hole extending longitudinally therethrough, the internally threaded surface within the coil receiving portion having an axial groove therein extending along its entire length, the groove having a depth greater than the depth of the thread and subtending an angle of between 60* and 120*, the bottom of the groove being generally concentric to the internally threaded surface of the coil form; and a thin flat resilient strip positioned within the groove, the strip having a width that substantially occupies but is less than the width of the groove at the grooves widest point and being of a length that exceeds the length of the groove, and being of a thickness that is less than the distance that the depth of the groove exceeds the depth of the thread, the strip forming a chord that intersects a continuation of the root of the internal thread of the coil form.
 6. In combination; a cylindrical member having an internally threaded center hold extending longitudinally therethrough, the internally threaded surface having an axial groove therein extending along its length, the groove having a depth greater than the depth of the thread; and a thin flat resilient strip positioned within the groove, the strip having a width that is less than the width of the groove at the grooves widest point and having a thickness that is less than the distance that the depth of the groove exceeds the depth of the thread, the strip forming a chord that intersects a continuation of the root of the internal thread of the cylindrical member. 